(1) Field of the Invention
This invention relates to an electrical insulating oil and oil filled electrical appliances. More particularly, the invention relates to an electrical insulating oil which is stable in refining treatments and which can maintain its excellent electrical characteristics even under high temperature conditions, and oil filled electrical appliances that are impregnated with the same.
(2) Description of the Prior Art
Electrical appliances such as oil-filled capacitors, oil-filled power cables and transformers have recently been made to withstand high electric voltages while being made small in size. With this tendency, various kinds of plastic films are used together with or in place of conventional insulating paper.
In the conventional art, refined mineral oils, polybutenes, alkylbenzenes, polychlorinated biphenyls and the like are used as electrical insulating oils; however, they have several drawbacks. For example, the use of halogenated aromatic hydrocarbons such as polychlorinated biphenyls was discontinued because it constitutes a public health hazard. Furthermore, the conventional electrical insulating oils are not satisfactorily compatible with the plastic materials such as polyolefin which are recently used in oil-filled electrical appliances.
With the requirements of durability to high-voltage and size reduction, it is necessary that the electrical insulating oil has a high dielectric breakdown voltage and a good hydrogen gas absorbing capacity.
The hydrogen gas absorbing capacity indicates the stability of the insulating oil against corona discharge (partial discharge) under high electric voltage conditions. The higher the hydrogen gas absorbing capacity, the smaller the likelihood of corona discharge, which leads to the advantage of the insulating oil having excellent stability or durability.
Meanwhile, in order to meet the requirement of high-voltage use, plastic materials such as polyolefin, polyester and polyvinylidene fluoride are used to replace either partially or completely the conventional insulating paper as insulating materials or dielectric materials for electrical appliances such as oil-filled electric cables and capacitors. In view of their dielectric strength, dielectric loss tangent and dielectric constant, polyolefin films, especially polypropylene and polyethylene films, are preferred as the plastic films.
When these plastics, especially the polyolefins such as polypropylene, are impregnated with insulating oils, some oils cause the films to swell or dissolve to some extent. If a plastic material is swollen, the thickness of insulating layers increases and the resistance to the flow of insulating oil increases in electrical cables, and insufficient impregnation with insulating oil occurs in electric capacitors, causing the formation of voids (unimpregnated portions), the undesirable lowering of the corona discharge voltage and the increase of the volumes of capacitors, all of which are not desirable.
In connection with the above-mentioned conventional electrical insulating oils, the values of dielectric breakdown voltages (BDV) and dielectric loss tangents (tan .delta.) are satisfactory to a certain extent, but the hydrogen gas absorbing capacity or corona discharge characteristic and the stability of the dimensions of plastic films are not satisfactory.
As described above, the requirements in the use of electrical insulating oils in recent years have become so severe that even a trace quantity of impurity in an electrical insulating oil sometimes causes a problem. Accordingly, it is necessary to subject electrical insulating oils to refining before they are used for impregnation of electrical appliances.
For the refining of electrical insulating oils, solid refining agents in granular or powder form are generally employed because they are efficient and the separation of them after refining is easy. The solid refining agents are exemplified by clays such as activated clay and fuller's earth; silica, silica gel, alumina, alumina gel, synthetic silica-alumina and activated carbon. The refining is done by adsorbing small quantities of impurities in an electrical insulating oil with these refining agents. The refined electrical insulating oil is used preferably just after refining though the refined oil can be used after storage.
These solid refining agents, however, have the Bronsted acid point or Lewis acid point as well as the adsorbing property. For this reason, many refining agents catalyze in chemical reactions. Accordingly, some ingredients in electrical insulating oils are chemically modified by the catalytic action of these solid refining agents in refining process, and thus stable and satisfactory refining cannot be done.
Furthermore, even though the oil-filled electrical appliances such as oil-filled capacitors are generally used at room temperature, they are sometimes used at considerably high temperatures due to climatic or other conditions in use. In large-sized capacitors, as the heat of dielectric loss is accumulated inside the capacitors, the temperature in the inner parts of capacitors sometimes becomes considerably high. Therefore, the uses under high temperature conditions must be taken into consideration with respect to oil-filled capacitors.
The interaction between electrical insulating oils and plastic materials at high temperatures are, however, different from the interaction at ordinary temperatures. The oil-filled capacitors impregnated with the above recently proposed electrical insulating oils are not always satisfactory at high temperatures, which is different from the uses at room temperature.
In U.S. Pat. No. 4,347,169 is disclosed an electrical insulating oil comprising diarylalkanes and unsaturated dimers or codimers of styrenes such as styrene and .alpha.-methylstyrene and oil-filled electrical appliances impregnated with the same. This electrical insulating oil is desirable because the compatibility with plastics is good, however, it has a defect that one of their component material such as unsaturated dimer of .alpha.-methylstyrene is liable to be deteriorated by solid refining agents. In addition, the oil-filled capacitors impregnated with such an electrical insulating oil cannot always exhibit their satisfactory performances under the use conditions at high temperatures such as 80.degree. C.